Abstract: We propose a new method for characterizing the
discontinuities in processes that are mostly continuous. We introduce a
causal operator that is used to specify when the value of a fluent has a
cause. A discontinuity in a fluent is allowed if the fluent's value
immediately after the discontinuity has a cause. The causal operator is
incorporated in a temporal logic with continuous time and real-valued
fluents. The resulting logic is a nonmonotonic logic suitable for
representing physical models of real world situations. We define a selection
function which given a set of models returns a subset of the models. This
selection function defines a nonmonotonic entailment operator. The intuitive
idea behind the selection function is that is should select all models where
all discontinuities are ``specified'' as allowed.